Outline, October 2CS-3013 A-term 20091Outline, October 2CS-3013 A-term Outline for October 2 Review:– Caches and Quantification of Cache Performance Virtual Memory Management (continued) Introduction to Protection and Security

Outline, October 2CS-3013 A-term Definition — Cache A small subset of active items held in small, fast storage while most of the items remain in much larger, slower storage

Outline, October 2CS-3013 A-term General Observation on Caching We create caches because There is not enough fast memory to hold everything we need Memory that is large enough is too slow Performance metric for all caches is EAT Effective Access Time Goal is to make overall performance close to cache memory performance By taking advantage of locality — temporal and spatial By burying a small number of accesses to slow memory under many, many accesses to fast memory

Outline, October 2CS-3013 A-term Definition – Effective Access Time (EAT) The average access time to memory items, where some items are cached in fast storage and other items are not cached… …weighted by p, the fault rate 0 ≤ p < 1 EAT = (1-p) * (cache access time) + p * (non-cache access time)

Outline, October 2CS-3013 A-term Goal of Caching To take advantage of locality to achieve nearly the same performance of the fast memory when most data is in slow memory I.e., solve EAT equation for p (1-p)*(cache_time) + p*(non_cache_time) < (1+x)*(cache_time) EAT x is “acceptable” performance

Outline, October 2CS-3013 A-term Goal of Caching (continued) Select size of cache and size of cache items so that p is low enough to meet acceptable performance goal Usually requires simulation of suite of benchmarks

Outline, October 2CS-3013 A-term Cache Applications Physical memory: cache of virtual memory I.e., RAM over disk TLB: cache of page table entries I.e., Registers over RAM Processor L2 cache: over RAM I.e., nanosecond memory over 10’s of nanoseconds Processor L1 cache: over L2 cache I.e., picosecond registers over nanosecond memory …

Outline, October 2CS-3013 A-term Cache Applications (continued) Recently accessed blocks of a file I.e., RAM over disk blocks Today’s airline flights I.e., local disk over remote disk

Outline, October 2CS-3013 A-term Questions?